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## Summary

Adds a bit more general background, intro to concepts, and guidelines
about what to use FF for and what not to


### Checklist

Delete any items that are not applicable to this PR.

- [ ] Any text added follows [EUI's writing
guidelines](https://elastic.github.io/eui/#/guidelines/writing), uses
sentence case text and includes [i18n
support](https://github.com/elastic/kibana/blob/main/packages/kbn-i18n/README.md)
- [ ]
[Documentation](https://www.elastic.co/guide/en/kibana/master/development-documentation.html)
was added for features that require explanation or tutorials
- [ ] [Unit or functional
tests](https://www.elastic.co/guide/en/kibana/master/development-tests.html)
were updated or added to match the most common scenarios
- [ ] [Flaky Test
Runner](https://ci-stats.kibana.dev/trigger_flaky_test_runner/1) was
used on any tests changed
- [ ] Any UI touched in this PR is usable by keyboard only (learn more
about [keyboard accessibility](https://webaim.org/techniques/keyboard/))
- [ ] Any UI touched in this PR does not create any new axe failures
(run axe in browser:
[FF](https://addons.mozilla.org/en-US/firefox/addon/axe-devtools/),
[Chrome](https://chrome.google.com/webstore/detail/axe-web-accessibility-tes/lhdoppojpmngadmnindnejefpokejbdd?hl=en-US))
- [ ] If a plugin configuration key changed, check if it needs to be
allowlisted in the cloud and added to the [docker
list](https://github.com/elastic/kibana/blob/main/src/dev/build/tasks/os_packages/docker_generator/resources/base/bin/kibana-docker)
- [ ] This renders correctly on smaller devices using a responsive
layout. (You can test this [in your
browser](https://www.browserstack.com/guide/responsive-testing-on-local-server))
- [ ] This was checked for [cross-browser
compatibility](https://www.elastic.co/support/matrix#matrix_browsers)


### Risk Matrix

Delete this section if it is not applicable to this PR.

Before closing this PR, invite QA, stakeholders, and other developers to
identify risks that should be tested prior to the change/feature
release.

When forming the risk matrix, consider some of the following examples
and how they may potentially impact the change:

| Risk | Probability | Severity | Mitigation/Notes |

|---------------------------|-------------|----------|-------------------------|
| Multiple Spaces—unexpected behavior in non-default Kibana Space.
| Low | High | Integration tests will verify that all features are still
supported in non-default Kibana Space and when user switches between
spaces. |
| Multiple nodes—Elasticsearch polling might have race conditions
when multiple Kibana nodes are polling for the same tasks. | High | Low
| Tasks are idempotent, so executing them multiple times will not result
in logical error, but will degrade performance. To test for this case we
add plenty of unit tests around this logic and document manual testing
procedure. |
| Code should gracefully handle cases when feature X or plugin Y are
disabled. | Medium | High | Unit tests will verify that any feature flag
or plugin combination still results in our service operational. |
| [See more potential risk
examples](https://github.com/elastic/kibana/blob/main/RISK_MATRIX.mdx) |


### For maintainers

- [ ] This was checked for breaking API changes and was [labeled
appropriately](https://www.elastic.co/guide/en/kibana/master/contributing.html#_add_your_labels)
- [ ] This will appear in the **Release Notes** and follow the
[guidelines](https://www.elastic.co/guide/en/kibana/master/contributing.html#kibana-release-notes-process)

---------

Co-authored-by: Alejandro Fernández Haro <alejandro.haro@elastic.co>
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Expand Up @@ -20,56 +20,53 @@ One example of invalid use cases are settings used during the `setup` lifecycle
if an HTTP route is registered or not. Instead, you should always register the route, and return `404 - Not found` in the route
handler if the feature flag returns a _disabled_ state.

In summary, Feature flagging is best suited for
- Phased rollout of the features (either to a specific customer, a subset of customers, or a % of overall users)
- Feature experimentation

Feature flagging is NOT suitable for
- Applying feature availability for licensing and/or tiers
- Restricting or applying customer entitlement to specific GA features

For a code example, refer to the [Feature Flags Example plugin](https://github.com/elastic/kibana/blob/main/examples/feature_flags_example/README.md)

## Registering a feature flag
## Key concepts

> [!IMPORTANT]
> At the moment, we follow a manual process to manage our feature flags. Refer to [this repo](https://github.com/elastic/kibana-feature-flags) to learn more about our current internal process.
> Our goal is to achieve the _gitops_ approach detailed below. But, at the moment, it's not available, and you can skip it if you want.
### Feature Flag

Kibana follows a _gitops_ approach when managing feature flags. To declare a feature flag, add your flags definitions in
your plugin's `server/index.ts` file:
A config key that defines a set of [variations](#variations) that will be resolved at runtime when the app calls [the evaluation APIs](https://docs.elastic.dev/kibana-dev-docs/tutorials/feature-flags-service#evaluating-feature-flags). The variation is decided at runtime based on static binary state (ON -> `variationA` vs. OFF -> `variationB`), or via [evaluation/segmentation rules](#evaluationsegmentation-rules).

```typescript
// <plugin>/server/index.ts
import type { FeatureFlagDefinitions } from '@kbn/core-feature-flags-server';
import type { PluginInitializerContext } from '@kbn/core-plugins-server';

export const featureFlags: FeatureFlagDefinitions = [
{
key: 'myPlugin.myCoolFeature',
name: 'My cool feature',
description: 'Enables the cool feature to auto-hide the navigation bar',
tags: ['my-plugin', 'my-service', 'ui'],
variationType: 'boolean',
variations: [
{
name: 'On',
description: 'Auto-hides the bar',
value: true,
},
{
name: 'Off',
description: 'Static always-on',
value: false,
},
],
},
{...},
];

export async function plugin(initializerContext: PluginInitializerContext) {
const { FeatureFlagsExamplePlugin } = await import('./plugin');
return new FeatureFlagsExamplePlugin(initializerContext);
}
```
### Variations

All the potential values that a feature flag can return based on the [evaluation rules](#evaluationsegmentation-rules). A feature flag should define at least 2 variations: the ON and the OFF states. The typical use case sets the OFF state to match the `fallback` value provided to [the evaluation APIs](https://docs.elastic.dev/kibana-dev-docs/tutorials/feature-flags-service#evaluating-feature-flags), although it's not a hard requirement and Kibana contributors might require special configurations.

### Evaluation/Segmentation rules

Set of rules used to evaluate the variation to resolve, based on the [evaluation context](#evaluation-context) provided by the app.

The rules can vary from a percentage rollout per evaluation context, or more complex IF...THEN filters and clauses.

Refer to the [Evaluation Context guide's examples](https://github.com/elastic/kibana-feature-flags/blob/main/docs/evaluation-context.md#examples) for some typical scenarios.

### Evaluation Context

Kibana reports a set of properties specific to each ECH deployment/Serverless project to help define the [segmentation rules](#evaluationsegmentation-rules). A list of the currently reported context properties can be found in the [Evaluation Context guide](https://github.com/elastic/kibana-feature-flags/blob/main/docs/evaluation-context.md).

### Feature Flag Code References

In the Kibana repo we run a [GH Action](https://github.com/elastic/kibana/actions/workflows/launchdarkly-code-references.yml) that links existing Feature Flags to their code references. This helps us figure out which flags have been removed from the code and, which ones are still being used.

> :information_source: New flags might take a few moments to be updated with their code references.
> The job runs on every commit to the `main` branch of the [Kibana repository](https://github.com/elastic/kibana), so the wait can take from a few minutes to a few hours, depending on the Kibana repo's activity.
## Registering a feature flag

After merging your PR, the CI will create/update the flags in our third-party feature flags provider.
At the moment, we follow a manual process to manage our feature flags. Refer to [this repo](https://github.com/elastic/kibana-feature-flags) to learn more about our current internal process.
Our goal is to achieve a _gitops_ approach eventually. But, at the moment, it's not available.

### Deprecation/removal strategy

When your code doesn't use the feature flag anymore, it is recommended to clean up the feature flags when possible.
When your code doesn't use the feature flag anymore, it is recommended to clean up the feature flags.
There are a few considerations to take into account when performing this clean-up:

1. Always deprecate first, remove after
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